1. Field of the Invention
The present invention relates to a multipole lens and also to a method of fabricating it.
2. Description of Related Art
In imaging apparatus, such as a scanning electron microscope, when a specimen is irradiated with an electron beam that is a charged-particle beam, aberration in the beam is corrected in order to image the specimen appropriately. In such an imaging apparatus, a multipole lens is mounted as an aberration corrector for correcting aberration in the electron beam.
A known structure of this multipole lens is described, for example, in JP2-230647, where plural (e.g., 8 or 12) polar elements are mounted. The polar elements are supported by an annular holding member and by a yoke disposed outside the holding member.
The multipole lens described in JP2-230647 has plural polar elements, a beam tube, and an annular yoke disposed outside the beam tube. Each of the polar elements consists of a mounting rod and a magnetic polepiece coupled to the front end of the rod. The beam tube is provided with airtight holes through which the mounting rods of the polar elements are passed.
Each polar element is fabricated by screwing the front-end portion of the mounting rod into the magnetic polepiece or by adhesively bonding or welding the front-end portion to the polepiece. The mounting rod and polepiece constituting the polar element are made of a magnetic material. The beam tube is made of an electrically insulating material. A metal coating is formed around each airtight hole.
The mounting rod of each polar element is firmly connected to the yoke at its base-end portion. In particular, the mounting rod is passed through the corresponding hole in the yoke via an insulator such that the rod is aligned along a straight line. Under this condition, the rod is firmly connected to the yoke. The mounting rod is airtightly and rigidly mounted within the airtight hole in the tube by welding that is done via the metal coating. Consequently, the weldment forms a sealably enclosed body.
A coil is mounted in the portion located between the beam tube of the mounting rod and the yoke. The polepiece joined to the front-end portion of the mounting rod is magnetically energized by passing electrical current through the coil.
A method of fabricating a multipole lens having previously-shaped front end forms by molding multipolar elements from a resin and a structure of the multipole lens are known as described, for example, in JP2005-19071.
The multipole lens described in JP2005-19071 has polar elements, an annular holding member for holding the polar elements, and an annular yoke disposed outside the holding member. Each polar element consists of a magnetic polepiece and a support rod. Each polar element is made up of the support rod and the magnetic polepiece mounted to the front end of the rod. The support rod and polepiece are made of a magnetic material, such as Permalloy or iron.
The held portions of the support rod which constitute the polar elements are inserted into through-holes formed in the holding member that is made of a nonmagnetic material, such as brass or phosphor bronze. Seal members, such as O-rings, are disposed at both ends of each through-hole. Inside each through-hole, the space between the outer surface of the held portion of the support rod and the inner surface of the through-hole is filled with a resin. The resin is cured inside the space formed in the through-hole. Consequently, the held portion of the support rod is firmly mounted inside the through-hole formed in the holding member via the cured resin. As a result, the polar elements are placed in position relative to the holding member at the held portions. Under this condition, the polar elements are fixed.
When a coil that is a heating element is placed in a vacuum, the ambient pressure is increased or the amount of hydrocarbons in the residual gas is increased. To prevent this phenomenon, the coil must be placed on the atmospheric side. Therefore, the coil is so disposed that its core is placed on the atmospheric side of the polar elements extending through the vacuum partition wall.
The vacuum partition wall has such a shape that it provides an assembly reference in stacking plural multipolar elements while securing positional accuracy among them. For example, this assembly reference provides a shape for securing coaxiality with other multipolar elements. A shape convenient for obtaining coaxiality is a circular form. This assembly reference needs to have a high positional accuracy with the shape of the front end of each polar element.
Shaping the front end of each polar element by wire electric discharge machining is convenient to uniformly machine the shape of each polar element, taken across an arbitrary cross section perpendicular to the optical axis. Furthermore, electric discharge machining needs to be used to machine a workpiece without producing distortion in it. From these viewpoints, the shape of the front end of each polar element needs to be machined by wire electric discharge machining.
However, machining of a circular assembly reference using wire electric discharge machining is impossible to perform because of interference with the polar elements on the atmospheric side. When the workpiece is reinstalled on the machine tool for plural machining steps, machining errors will be accumulated. To prevent this accumulation, the workpiece is ideally machined in a single machining step.
Because of the reasons described so far, in the prior-art technique, the step for machining an assembly reference cannot be made common with the step for machining of the front end of each polar element. Therefore, it is necessary that alignment be made with a previously-shaped assembly reference and that the front end of each polar element be machined. The positioning error is added to the error produced between the assembly reference and the shape of the front end of each polar element. For example, accumulation of these errors will deteriorate the following kinds of accuracy:                1) coaxiality between the center of the multipolar element and the circular shape of the assembly reference,        2) orthogonality between the assembly reference end surface of a single stage of multipolar element and the center axis of the multipolar element.        Where polar elements whose front ends have been previously shaped accurately are assembled, the following kinds of accuracy are deteriorated in addition to the kinds of accuracy 1) and 2) described above:        3) flatness of the end surfaces of the polar elements.        
That is, the individual polar elements become nonuniform in the end surface position. There are other problems. Where a blank member shaped into polar elements is adhesively bonded to a metal member (JP2005-19071), it is difficult to determine the position of the blank member because of positional deviation occurring during the adhesive bonding. The blank member is a workpiece to be machined by electric discharging. Where a blank member is brazed to a ceramic material (JP2-230647), it is impossible to machine ceramic assembly references by electric discharging.